There are no CB Activity language strings yet. Once CB Activity has been upgraded it'll include its own language files like the other plugins that have been upgraded. To translate it now you need to create language files for it or you need to add language strings for it to your CB language plugin.

Do you mean that all plugins language files are now in language.php ? including paid subscriptions and groupjive ?

No, each plugin has its own language strings. CBs default_language contains strings for core CB and nothing else.

Or do we keep cbteamplugins_language ?

It doesn't load anymore. You can migrate over your language strings as needed then delete it.

That's crazy that you make such changes between 2.0.3 and 2.0.4 without warning no body !!
This is really a big change for customers !!!

Loading of that file was legacy support, but we found it was breaking the language loading. So we had to discontinue loading of it. All our transifex projects have been up to date with the appropriate file structure for quite some time.

So can you explain with 2.0.4 where to put language files for CB activity in french ? and for other plugins ? do we have to set a language folder somewhere inside the plugin folder ?

CB language plugins should have a language.php, which all of your frontend language files go. For French you can use the following and adjust any further strings as needed.

forge.joomlapolis.com/projects/lan-cb-fr/files

See the language plugin under "CB 2.0+ Language Plugins" for CB 2.x French language plugin.

Each CB plugin should have its own language file inside of the language plugins folder. So for example you'd create the following, which would load when CB Activity loads.

fr-FR/cbplugin/cbactivity-language.php

Nick has also provided a language override tutorial, which you can find below to override strings.

www.joomlapolis.com/forum/6-news-and-publicity/227556-cb-language-override-video-tutorial-resources#254320

Is it the same thing for cbpaidsubscription and groupjive ?

CBSubs and GroupJive both still have their own language plugins, but technically they are not needed anymore and you can use the above described method. GroupJive language plugins will be deprecated in its major upgrade.

do they have to be in the cbplugin folder already ?

No, you can create them as needed using language.php as an example (just empty the arrays strings and begin adding your own). Once we've all our plugins upgraded all of this mostly becomes automated as we can generate them from our transifex.

There is a CB Activity transifex project here:

www.transifex.com/projects/p/cbactivity/

You can join the French project and translate.

Every Monday we grab all translations (that are 100%) and generate new CB Language plugins in our forge area:
forge.joomlapolis.com/projects/lan-cb